There are a variety of image processing methods for detecting a local motion of a 3D object. For example, there are image processing methods of handling a face being a 3D object, as a target and detecting a pose of the face itself, a motion of each part of the face, etc. from a motion picture of the face taken by a camera. Such image processing methods include those of detecting a gaze of eyes (cf. Non-patent Document 1: J. Heinzmann and A. Zelinsky, “3-D facial pose and gaze point estimation using a robust real-time tracking paradigm,” IEEE Int. Workshop on Automatic Face and Gesture Recognition, pp 142-147, 1998). Non-patent Document 1 describes a method of taking a picture of a face with a monocular camera and determining the gaze while tracking the face from the motion picture thereof The method described in Non-patent Document 1 is to find a number of feature points of the face by Kalman filtering and to obtain the facial pose and gaze using the found feature points.